1. Field of the Invention
The present invention relates to a reach type forklift truck provided with a fork for lifting a load, the fork is movable back and forth.
2. Description of the Related Art
Conventionally, as a forklift truck for delivering a load, there is known such a reach type forklift truck as shown in FIG. 3. A reach type forklift truck, which is designated by reference numeral 1 in FIG. 3, includes a truck main body 2, a pair of straddle arms 3 respectively provided integrally with and extended forwardly from the truck main body 2, a mast 4 mounted between the pair of straddle arms 3 so as to be movable back and forth, and a fork 5 mounted on the mast 4 movably so as to be raised and lowered. On the bottom portion of the truck main body 2, there is disposed a drive wheel 6 that drives the truck. In the front end portions of the straddle arms 3, there are disposed a pair of load wheels 7 for supporting the weight of the load.
On the two straddle arms 3, more specifically, on the mutually opposed sides thereof, there are disposed a pair of reach rails 3a, as shown in FIG. 4. Each of reach rails 3a is formed in a substantially U-like shape, and the reach rails 3a are mounted in such a manner that their respective openings face the inside of the truck main body 2. A guide roller 8 mounted on the mast 4 is rollably fitted into the reach rails 3a through their openings.
By the way, in the thus structured conventional reach type forklift truck 1, slippage is detected, by detecting the difference between the drive wheel's rotation speed and the load wheel's rotation speed, for example.
In order to detect the rotation of the load wheels 7, as shown in FIGS. 4 and 5, a detect disk 9 including a large number of slits S formed in the outer peripheral edge portion thereof is coaxially mounted on the inside of the load wheel 7. Also a pickup sensor 10 is disposed on the lower portion of the reach rail 3 so as to be opposed to the slits S of the detect disk 9 to detect the slits S. The rotation speed of the load wheel 7 can be detected by detecting the slits S using the pickup sensor 10.
The pickup sensor 10 is mounted on a bracket 11 that is hang down from the lower portion of the reach rail 3a. 
In the above-mentioned conventional reach type forklift truck 1, there are still left the following problems to be improved. That is, to detect the rotation of the load wheels 7, it is necessary not only to mount the detect disk 9 including a large number of slits S on the load wheels 7 but also to mount the pickup sensor 10 for detecting the slits S on the lower portion of the reach rail 3a via the bracket 11. Since many parts are required and machining of the detect disk 9 is troublesome, manufacturing costs of the forklift truck 1 is increased.
Also, in the conventional reach type forklift truck, since the gap between the lower surface of the reach rail 3a and the traveling road surface of the forklift truck 1 is narrow, there is an inconvenience that the bracket 11 collides with the uneven road surface or fallen objects such as stones. In order to avoid such inconvenience, the shape of the bracket 11 to be mounted on the reach rail 3a must be reduced in size, so that the shape and size of the pickup sensor 10 is limited.
In JP-A-2001-302198, there has been proposed an apparatus for detecting the number of rotations of a driven wheel of a reach type forklift truck. In a rotation detector disclosed as an embodiment in the publication, a sensor for detecting the number of rotations of the tire is mounted on a lower surface of a reach rail, and the sensor is protected by a guard (refer to FIGS. 1 and 2 in the publication). In addition, in a rotation detector disclosed as another embodiment in the publication, such a sensor is mounted on an axle and a detected portion is disposed on an inner circumference of the wheel at a position confronting the detected portion (refer to FIG. 9 in the same publication).
On the other hand, in detecting the number of rotations of a rotating body, there has been tried a method for detecting the number of rotations of a bearing which supports the rotating body rather than detecting directly the number of rotations of the objective rotating body, and there has been proposed a bearing on which a rotation detector is provided (JP-A-6-81833).
One example of the rotation detector disclosed in JP-2001-302193, the sensor is situated on the under surface of the reach rail and the sensor so situated is then protected by the guard, the sensor is protected by the guard from a direct collision with fallen objects. However, it is desirable for the sensor to be mounted and maintained in a condition in which neither collision nor other impacts can be applied to the sensor in order to maintain the required accuracy, and even if the sensor is protected by the guard, when considering the possibility that an impact applied to the guard is transmitted to the sensor via the reach rail, the accuracy of the sensor is not necessarily secured at a sufficient level. In addition, since mounting the guard is troublesome, the costs are increased.
In contrast, another example of the rotation detector disclosed in JP-A-2001-302193, the sensor is mounted on the axle and it is possible to eliminate a part such as the guard which protrudes outwardly and therefore from the viewpoint of maintaining the accuracy of the sensor, there seems to be no problem. On the contrary, as is described in the publication, this construction can become effective only in a case where a brake system is not mounted on the wheel.
In addition, in the bearing according to JP-6-81833 on which the rotation detector is provided, since the rotation of the inner race is detected by the sensor provided on the outer race, the rotating body needs to be fitted in the inner race. Namely, the rotating body which is an objective for detection is limited to an axle which is fitted in the inner race of the bearing or a rotating body which is adapted to rotate together with the axle. Due to this, this construction cannot be used for a driven wheel of a reach type forklift truck which is provided on the outer race of the bearing whose inner race is fixed to the axle.